CVE-2021-41039 in Mosquitto
Summary
by MITRE • 12/01/2021
In versions 1.6 to 2.0.11 of Eclipse Mosquitto, an MQTT v5 client connecting with a large number of user-property properties could cause excessive CPU usage, leading to a loss of performance and possible denial of service.
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Analysis
by VulDB Data Team • 12/08/2021
The vulnerability identified as CVE-2021-41039 affects the Eclipse Mosquitto MQTT broker software within versions 1.6 through 2.0.11, representing a significant security concern that impacts the operational integrity of MQTT-based communication systems. This flaw specifically targets the processing of user-property attributes within MQTT version 5.0 protocol implementations, where an attacker can exploit the broker's handling of these properties to consume excessive computational resources. The vulnerability stems from inadequate input validation and resource management within the broker's parsing logic for MQTT v5 client connections, creating a scenario where malicious or malformed client requests can trigger disproportionate system resource consumption.
The technical implementation of this vulnerability involves the broker's processing of user-property key-value pairs that are part of the MQTT v5 protocol specification. When an MQTT v5 client establishes a connection and includes an excessive number of user-property entries, the Mosquitto broker's internal parsing mechanism becomes overwhelmed with processing overhead. This occurs because the broker does not implement proper bounds checking or resource limiting mechanisms for the number of properties that can be processed during client connection establishment. The flaw is categorized under CWE-770, which addresses allocation of resources without proper limits, and represents a form of resource exhaustion attack that specifically targets CPU processing capabilities rather than memory or network bandwidth.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially enable full denial of service conditions within MQTT broker environments. Systems relying on Mosquitto for critical IoT communications, industrial automation, or smart city infrastructure could experience complete service interruption when subjected to this attack vector. The excessive CPU usage pattern typically manifests as sustained high processor load that can persist for extended periods, making the service unavailable to legitimate clients while the malicious connection attempts are active. This vulnerability affects any organization using Mosquitto as an MQTT broker in production environments, particularly those with high client connection volumes or those that do not implement proper monitoring and rate-limiting controls.
Organizations can mitigate this vulnerability through several approaches that align with established cybersecurity frameworks and best practices. The primary mitigation strategy involves upgrading to Mosquitto version 2.0.12 or later, which includes specific fixes for the user-property processing logic. Additionally, implementing connection rate limiting and property count restrictions within broker configuration files can provide defense-in-depth measures that limit the impact of potential exploitation attempts. Network-level controls such as implementing connection throttling, monitoring for unusual property count patterns, and deploying intrusion detection systems can help identify and block malicious connection attempts. From an ATT&CK framework perspective, this vulnerability aligns with techniques related to resource exhaustion and service disruption, specifically targeting the availability aspect of the CIA triad while potentially enabling further exploitation opportunities through system instability.